The cardiac conducting system consists of specialised areas of cardiac muscle found in the sinoatrial node (SA node), atrioventricular node (AV node) and the bundle of His.
SA node:
The SA node is located in the right atrial wall between the crista terminalis and the opening of the superior vena cava. Contraction of the cardiac muscle is initiated by the SA node, with the electrical depolarisation spreading quickly to the right and left atria and the AV node. The SA node receives both parasympathetic and sympathetic fibres of the autonomic nervous system, which act to alter the rate of autorhythmicity.
AV node:
The AV node is located in the floor of the right atrium near the interatrial septum, medial to the ostium of the coronary sinus and above the septal cusp of the tricuspid valve. This allows atrial depolarisation to spread over right and left atria before reaching the AV node. As a result, the atria contract before the AV node is stimulated and the depolarisation conducted down the bundle of His to the ventricles.
AV bundle or bundle of His:
This arises from AV node and passes through the annulus fibrosus. It then descends on the posterior border of the membranous part of the interventricular septum to enter its muscular part. The bundle then splits into two branches, called the left and right bundle branches (crura) to conduct electrical activity to the ventricles. They consist of Purkinje fibbers which have a rapid conduction rate (low latency).
Figure.10: The conducting system (Drake et al, 2009).